Image data collected from an incoherently illuminated scene (for example, image data collected using ambient light from the sun) tends to be degraded in the presence of phase and amplitude aberrations. Aberrations may arise from a variety of sources, such as optical design residual, optical-fabrication error, misalignment among optical elements, degradations in reflectivity or transmisivity of portions of optical elements and atmospheric turbulence.
As a result of aberrations, images created from collected image data in the presence of unknown aberrations may be blurred or otherwise degraded, resulting in loss of resolution, loss of contrast and reduction in interpretability. Previous imaging techniques have attempted to overcome the degrading effects of aberrations in acquired image data. One of these approaches is known as the method of phase diversity and is described in U.S. Pat. No. 4,309,602 to Gonsalves, et al., entitled “Wavefront-Sensing by Phase Retrieval.” This technique involves collecting two images of an object in the presence of unknown aberrations, with one of the two images being degraded by a known amount of defocus and the other image being a focused image. The defocus of one image during data collection creates phase diversity between the two images. The two images are then processed to determine unknown atmospheric phase aberrations by identifying a combination of the object and phase aberrations consistent with the collected images, given the known amount of defocus. Thereafter, the system may be adaptively corrected to eliminate or minimize the phase aberrations in the received imagery.
Another technique previously used to overcome the degrading effects of aberrations in acquired image data is known as “Measurement-Diverse Speckle Imaging”, which is disclosed in U.S. Pat. No. 5,384,455 to Paxman. This technique involves collecting a sequence of two or more pairs of short-exposure images of an object in the presence of unknown aberrations, with each pair of images having measurement-diversity. An iterative process may then be employed to jointly estimate the object that is common to all collected images and the unknown phase aberrations associated with each image pair.
However, as stated above, images collected from an incoherently illuminated scene may be degraded by both phase and amplitude aberrations. Prior attempts at accounting for aberrations in acquired image data may account for phase aberrations in the collected image data, but they fail to account for any amplitude aberrations. As a result, the image created from the data collected by these systems may be degraded due to amplitude aberrations and a substantially diffraction-limited image of the object may not be obtained.
Therefore, there is a need for a system, method and algorithms capable of imaging extended objects which account for both phase and amplitude aberrations caused by the atmosphere and/or the system used for obtaining the image data.